This application claims the priority benefit of Canadian Patent Application No. 2,326,540 filed on Nov. 16, 2000 as file no. 37304.2 and entitled Method of Treating Industrial Wastewater.
The present invention relates to methods of treating industrial wastewater comprising pulp and paper mill effluents involving physico-chemical precipitation.
Large amounts of water are used in the various stages of the papermaking process in a pulp and paper mill (xe2x80x9cPPMxe2x80x9d). The papermaking process includes several steps, i.e., bark removal, pulping, bleaching, etc. Each of these steps uses a great deal of water. While significant improvements have been made in conserving and reusing water in the papermaking process, it is still necessary to discharge a certain amount of wastewater from the system.
The wastewater from a pulp mill is contaminated with lignins, lignin degradation products, cellulose and holocellulose degradation products and humic acids. These contaminants make the effluent stream dark colored and are often referred to as color bodies. Since pulp mill plants produce large quantities of this densely-colored effluent, the discharge of this effluent into adjacent streams and bodies of water can cause an objectionable discoloration and pollution of the water.
The effluent stream leaving the pulp plant usually empties into a settling basin. Various processes have been proposed for the decolorization of the effluent at this stage. Conventional effluent treatment processes, such as the precipitation of the suspended solids with lime, polyelectrolyte polymers or inorganic metallic salts are effective in removing some color from such effluent. However, polyelectrolyte polymers are prohibitively expensive for treating the quantities of effluents generated by commercial-size pulp and paper plants. Furthermore, inorganic metallic salts produce a great deal of sludge when used at appropriate levels to treat this quantity of water. This sludge must be removed and disposed of at a cost which renders these metallic salts impractical.
Therefore, there is a need in the art for an efficient process of removing colour bodies and organic pollutants from a PPM effluent, which process is not prohibitively expensive and which may successfully remove such colour bodies and organic pollutants at least as effectively as the prior art methods.
The present invention is directed to methods of treating a wastewater stream to remove contaminants. The invention is described herein with reference to the effluent from a PPM and in particular, from a chemical thermomechanical pulping (xe2x80x9cCTMPxe2x80x9d) process. However, it is to be understood that the present invention may be applied to other wastewater streams where efficient contaminant removal is desired. For example, the present invention may be applied to other PPM effluents such as BCTMP, DIP, Kraft, SCMP, TMP, fine papers, roofing felt, sulphite and other industrial wastewater streams.
Therefore, in one aspect, the invention comprises a method of removing contaminants from a wastewater stream comprising the steps of:
(a) obtaining a plurality of wastewater samples from the wastewater stream;
(b) choosing a plurality of operating variables, at least one of which is concentration of a polymer comprising casein;
(c) operating a plurality of trials to assess treatment efficiency wherein each trial is operated using a different combination of operating variables from other trials;
(d) determining an optimum value of each operating variables to achieve optimum contaminant removal from the wastewater stream; and
(e) treating the wastewater stream with a polymer comprising casein utilizing the optimized values determined in step (d).
In one embodiment, the wastewater stream is the effluent stream of a wood pulp bleaching process.
In one embodiment, the determination of optimum values is carried by a statistical analysis. The method of statistical analysis may be the Box-Wilson method or Central Composite Design (xe2x80x9cCCDxe2x80x9d) method.
In a preferred embodiment, at least 5 operating variables are chosen and in a particularly preferred embodiment, the at least 5 operating variables are the following:
(a) concentration of the polymer comprising casein;
(b) pH;
(c) temperature;
(d) hydraulic retention time; and
(e) degree of agitation.
Of course, in alternative embodiments, the operating variables may be limited to one or more operating variables which may or may not include those specific variables listed above.